Absorption of radio frequency energy



March 1957 A. R. HAMILTON ABSORPTION OF RADIO FREQUENCY ENERGY FiledAug. 24, 1953 2 Sheets-Sheet 1 INVENTOR ALLEN R. HAM/L TON ATTOkNEYMarch 1957 A. R. HAMILTON ABSORPTION 0F RADIO FREQUENCY ENERGY 2Sheets-Sheet 2 Filed Aug. 24, 1953 RESONANT CIRCUIT OUTPUT WITH I% ITENERGY ABSORBED RESONANT CIRCUIT OUTPUT WITH :1 D r m r G 0 m m N N 7 R0R0 k 0/ 0 T T 0 P T T TR R NO NO 55 ES L m m M U U G C c sww 5 3 TR TRfi MM MM r b PE o m LF N w E M A R A R T D w m i E v Ar IN T 0 E m m mwE 6c DI s 2 I E H ZERO Pf ENERGY ABSORBED MN EO n P R 0 MA My R6 U.R M 5tr an FIG. 3.

GRID B/ASED FAR BEYOND PLATE CURRENT CUT-OFF 35 RESONANT CIRCUIT OUTPUTWITH 36 RESONANT CIRCUIT OUTPUT WITH ff. ENERGY AESORBED A T TORNE Y 1ZERO Rf. ENERGY ABSORBED United States Patent ABSORPTION OF RADIOFREQUENCY ENERGY Allen R. Hamilton, Rochester, N. Y., assignor, by mesneassignments, to Consolidated Electrodynamics Corporation, Pasadena,Calif., a corporation of California Application August 24, 1953, SerialNo. 376,222

8 Claims. (Cl. 324-61) This invention relates to electrical apparatusfor measuring the amount of R. F. energy absorbed by a sample from aresonant circuit. More specifically it provides an improved circuit ofthe Q-meter type with greatly increased sensitivity for accuratemeasurement of minute changes of Q in a resonant circuit due to thepresence of the sample in inductive relationship with the circuit. Itfiinds its principal application in measuring the concentration of aknown material dissolved or dispersed in the sample.

The Q of a resonant circuit is a figure of merit assigned to thecircuit, and it is commonly defined by the follov ing equation:

where X =the reactance of the circuit R=:the resistance of the circuit.

It is common practice to measure the Q of resonant circuits withconventional Q-meters consisting of a radio frequency generatorconnected to the resonant circuit whose Q is to be measured, and avacuum tube voltmeter for measuring voltages developed in the resonantcircuit. The voltage output of the radio frequency generator is known ormeasured, and the voltage developed across either a capacitor or aninductance in the resonant circuit is measured with the vacuum tubevoltmeter. The voltage developed across either component (capacitor orinductor) of the resonant circuit is equal to the product of the Q ofthe circuit and the voltage applied from the radio frequency oscillator.Thus the measurement of voltage developed across the capacitor in aseries resonant circuit, for example, permits ready calculation of the Qof the circuit from the equation:

where E1=the voltage applied by the R. F. generator to the resonantcircuit Ez=voltage measured across the capacitor.

An important feature of a Q-meter is the use of a vacuum tube voltmeterwhich is capable of measuring alternating voltages without requiring anyappreciable power from the voltage being measured. My invention providesan improved Q-meter circuit in which the sample under test is broughtwithin the radio frequency field existing in the resonant circuit so asto absorb R. F. energy from the circuit and thusinduce a change in the Qof the resonant circuit. The Q of a resonant circuit is proportional tothe ratio of the amount of R. F. energy 2,786,181 Patented Mar. 19, 1957stored in the circuit to the amount of R. F. energy dissipated in thecircuit during one cycle. The amount of radio frequency energy absorbedby the sample is a function of the composition of the sample, and theabsorption of radio frequency energy causes a lowering of the Q of thecircuit. As can be seen from equation (2), the lowering of the Q of thecircuit while the applied voltage from the R. F. generator is heldconstant will result in a reduced reading of the vacuum tube voltmeterconnected across one of the components of the resonant circuit. Thechange in the reading of the vacuum tube voltmeter is a function of theamount of R. F. energy absorbed by the sample and hence is a function ofthe composition of the sample.

Conventional Q-meters are satisfactory for measuring relatively largechanges in Q, but are not sufiiciently sensitive to detect therelatively small amounts of R. F. energy absorbed by many samples. Forexample, my experiments show that presently available Q-meters areincapable of detecting a concentration of hydrochloric acid in distilledwater to the amount of 1() moles per liter, yet my circuit is capable ofmeasuring this and much smaller concentrations. The insensitivity of theconventional Q-meter circuits is due to the fact that the vacuum tubevoltmeters which they employ are designed to operate within the normalvoltage ratings of the voltage detector tubes used. A small change inthe Q of the test circuit results in a proportionally small change inthe voltage appearing across the vacuum tube voltmeter terminals. Suchvoltage changes may be so small as to be undetectable in conventionalcircuits.

If a sample which is to be analyzed absorbs only a small portion (forexample, 1%) of the total R. F. energy stored in the resonant circuit,then the resultant voltage change as read on a conventional vacuum tubevoltmeter would be in the same proportion. Such a small change isundetectable with conventional instruments when attempting todistinguish between samples absorbing very small amounts of R. F.energy. My invention provides a means whereby changes in the R. F.energy absorbed by such samples in a series resonant circuit (and theresulting changes in circuit Q) can be easily detected and measured.This is accomplished by imposing an abnormally large negative bias onthe control grid of the V. T. V. M. detector tube. This permits theapplication of a large amount of R. F. energy to the resonant circuitwithout causing the control grid of the detector tube to be drivenpositive during the measurment. This is particularly important becauseif the control grid of a vacuum tube voltmeter is driven positive, it nolonger is a high impedance device of the type required for Q-metercircuits.

My invention contemplates the provision of an improved Q-meter circuitsuitable for measuring the amount of R. F. energy absorbed by a sampledisposed in an inductive relationship with a resonant circuit comprisingthe combination of a radio frequency generator capable of variable poweroutput, a resonant circuit connected to the output of the R. F.generator, means for tuning the resonant circuit, a vacuum tubevoltmeter connected across the resonant circuit and having a controlgrid and a plate circuit, means for biasing the control grid of thevacuum tube to a negative value beyond its normal cut-oil bias, acurrent meter connected in the plate circuit of the tube, means forvarying the power output of the R. F. generator sufficient to overcomethe negative bias on the vacuum tube and cause plate current to passthrough the meter to give a relatively large measurement with no samplein inductive relationship with the resonant circuit so that the platecurrent will decrease substantially when a sample which absorbs R. F.energy is placed in inductive relationship with the resonant circuit. Ihave found that satisfactory analysis can be made with the sampledisposed in the circuit to absorb R. F. energy from either the capacitoror inductor of the resonant circuit, and I use the term inductiverelationship to meaneither. of these two positions.

These and other aspects of the invention will be more thoroughlyunderstood 'in the light of the following detailed description taken inconjunction with the accompanying drawings in which:

Fig. l is a Wiring diagram showing a preferred form of the apparatus ofthe invention;

Fig. 2 illustrates operating conditions for a detector tube in a vacuumtube voltmeteremployed in a conventional Q-meter circuit; and

Fig. 3 illustrates operating conditions for the detector tube as used inthe present invention.

Referring to Fig. l, the principal elements or" the apparatus are aradio frequency generator iii, a resonant circuit 11, and a vacuum tubevoltmeter 12. Theradio frequency generator is a crystal oscillatorcircuit having a tuned-grid tuned-plate arrangement. Chi-5672 tetrodevacuum tube 13 is used as an oscillator tube. A crystal 14 in parallelwith a resistance R1 is connected between a control grid to and acathode 17 of the tube to make up the tuned-grid circuit. A tickler coil13 is connected in series with the tuned-grid circuit to insurecontinuous oscillation of the tube. A tank circuit 19 having an inductorLi, a variable capacitor C1 and a fixed capacitor C2 is connected inseries ina plate circuit 2% of the oscillator tube. The inductor ispositioned in inductive relationship with the tickler coil to insureadequate feedback to the grid of the tube. A resistor R2 and a capacitorC3 in series connect a point in the circuit between the inductor andfixed capacitor of the tank circuit to the ground to reduce the effectof R. F. voltage on the battery circuit. A rheostat R3 is connected inthe plate circuit of the oscillator tube to control theamount of powerdelivered by the R. F. generator by controlling the amount of currentsupplied to the plate circuit from a plate supply battery 21. quencygenerator is capacity-coupled to the resonant circuit through acapacitor C4. A capacitor C is connected across the output of the R. F.generator to provide for additional stability of R. F. voltage appliedto the resonant circuit. The resonant circuit consists of a variableinductor L2 in series with a capacitor Cs adapted to con tain a sampleholder 22 into which a sample is placed or through which a sample ispassed. For convenience the sample holder is a plastic tube or sleeve.The vacuum tube voltmeter consists of a detector tube 23 (CK-5672tetrode vacuum tube) connected in a plate circuit 24 to an ammeter 25.The normal cut-off bias of the detector tube is about -8 volts. Acondenser C7 is provided between the plate and a cathode of the tubeforby-passing of the plate circuit to alternating currents. The input ofthe vacuum tube voltmeter is a control grid-26 of the detector tube andis connectedacross the capacitor of the resonant circuit by anelectrical lead 27. A potentiometer R4, R5, Rs powered by a biasingbattery 23 is connected to the control grid of the detector tube, and isused to apply a large negative bias to the control grid. T he value ofthis negative bias is of the order of five to ten times greater than thenormal cut-oil bias of the vacuum tube, depending upon the degree ofsensitivity desired. The term normal cut-off bias is empioyed here tomean that negative bias required to be imposed on the control grid ofthe vacuum tube to prevent any current from flowing on theplate circuitof the vacuum tube when the vacuum tube is being operated at normalfilament and plate voltages. For the sake of simplicity the details ofthe various switches and filamentpower supplies of standard Q-metercircuits are not illus-- trated in Fig. 1. v

In order to obtain a clear idea of the values of the vari- The output ofthe radio freous components employed in the circuit, attention should bedirected to the following table:

mH. =microHenry.

Figs. 2 and 3 illustrate the advantage which the above circuit has overconventional Q-meter circuits. For the purpose of illustration it isassumed a sample which absorbs 1% of the R. F. energy applied to theresonant circuit is being analyzed. The tube characteristic curve shownin Fig. 2 illustrates operating conditions for a detector tube inavacuum tube voltmeter employedin a conventional Q-meter circuit. Underthe conditions illustrated in Fig. 2, the control grid is biased nearplate current cut-off 29. The resonant circuit output 30- with no R. F.energy absorption by'the sample produces a detector tube plate current31. The resonant circuit output'32 with energy absorption producesdetector tube plate 1. /0 current 33. It is apparent from thisillustration that absorption of 1% of the R. F. energy absorbed producesa small change in plate current.

Fig. 3 illustrates operating conditions for the detector tube as used inthe present invention. The control grid is biased at a value 34 farbeyond detector tube plate current cut-off point 2 The resonant circuitoutput 35 with no R. F. energy absorbed is adjusted to produce the sameplate current 31, as indicated in Fig. 2. The resonant circuit output 35with 1% of the R. F. energy absorbed by the sample produces a platecurrent 37. It is apparent that the variation in plate current forthe-case illustrated in Fig. 3 is much greater than that produced underthe conditions shown in Fig. 2, although the. percentage of R. F. energyabsorbed by the sample is the same.

The apparatus is operated as follows: a first calibrating samplecontaining a known concentration of the material which is to be measuredis placed in the sample holder which conveniently is a glass test tube.The first calibrating sample should contain the minimum concentration ofthe material which is anticipated to be measured subsequently. Thevariable inductance is then tuned to produce resonance for the frequencybeing supplied from the radio frequency generator. This is easilyaccomplished by' adjusting the variable inductance to give a maximumreading on the vacuum tube voltmeter. The most desirable frequency touse in themeasurement will vary with the type of sample being analyzed.For example, my experiments have shown that a frequency of 10 megacyclesis desirablein measuring a concentration of'about l0 to 10- normality ofhydrochloric acid in water, while a frequency of 1.5 megacycles is moresuitable for detecting the presence of weaker solutions in the order of1() normality. The power output from the radio frequency generator isthen set to give a relatively large reading (preferably full scaledefiectionlon the vacuum tube voltmeter. The first calibrating sample isthen removed from the sample holder and a second calibrating sample ofhigher but known concentration of the material is placed in the sampleholder. For example, the sample could contain the highest concentrationof material which is to bedetected. Usually the second calibratingsamplewill not have the same-effect on: theresonant frequency of theresonantcircuitull asthezfirst calibratingsample, and the variable inductor L2is used to re-tune the circuit to the frequency of theR. 'F. generator.The second calibrating sample absorbs more radio frequency energy fromthe resonant circuit than the prior sample and causes the Q of thecircuit to be lower, thus decreasing the reading of the vacuum tubevoltmeter. If the first calibrating sample represents a minimumconcentration of a material to be determined and the second calibratingsample represents the maximum of the material to be determined, then thedifference between the two respective vacuum tube voltmeter readingsshould be as large as possible to obtain maximum accuracy for subsequentanalyses of samples containing unknown concentrations of the material.If the initial setting of the circuit with the calibrating samplesresults in a spread of the vacuum tube voltmeter readings which is toosmall, it is increased by increasing the negative bias applied to thecontrol grid of the detector tube. The above calibrating procedure isthen repeated so that the power output of the radio frequency generatoris set to produce a maximum voltmeter reading when the calibratingsample with the lowest concentration of material is again placed in thesample holder. The first calibrating sample is removed from the sampleholder and the second calibrating sample is inserted in the sampleholder. If the bias on the control grid of the vacuum tube voltmeter hasbeen increased sufficiently, the spread of the voltmeter readings forthe two calibration samples will be substantially full scale. If thespread in readings is too large, i. e., the voltmeter reading with thesecond calibrating sample is less than the lowest value provided on thevoltmeter scale, the bias is too high, and the calibrating process mustbe repeated with a decreased bias on the control grid of the vacuumtube. In other words, by trial and error method using calibratingsolutions of the desired concentration range of the material to bemeasured, the proper bias for the control grid of the vacuum tubevoltmeter and the proper power output of the radio frequency generatorcan be determined to give a desired sensitivity. Once the desiredsensitivity is obtained a complete calibration over the entire range ofconcentration to be measured is made by noting the voltmeter reading foradditional calibrating samples of intermediate concentration, care beingtaken to retune the resonant circuit to the frequency of the R. F.generator each time a new calibrating sample is inserted in the sampleholder. In this way the amount of R. F. energy absorbed in each of thecalibrating samples is measured at a constant optimum frequency, i. e.the frequency of the R. F. generator.

The concentration of any unknown sample within the calibrating range isthen readily determined by placing the unknown sample into the sampleholder, tuning the resonant circuit to the frequency of the R. F.generator and comparing the vacuum tube voltmeter reading with thevalues obtained for the calibrating samples.

Although the preferred form of my invention is to power the circuit withbatteries so that the apparatus is readily portable, the same type ofmeasurement can be made with a circuit with a non-portable arrangementdrawing its power supply from power lines. It should also be pointed outthat similar analyses of samples can be made by placing the samples inthe inductance of the resonant circuit and using a variable capacitor totune the circuit as described above.

It should also be understood that the sample to be analyzed can beplaced in inductive relationship with the resonant circuit eitherstatically or dynamically. That is, the sample can be introduced andremoved batchwise, or it can be continuously flowed through the spaceadapted to contain the sample. For example, the sample may be flowedcontinuously through a glass or plastic pipe which is disposed ininductive relationship with its capacitor.

My invention has the advantages that the sample is analyzed with anon-destructive method, and it is not necessary to make physical contactwith the sample. This permits the analysis of the sample in a sealedcontainer without altering either the container or the sample.

My invention is applicable as a chemical analysis method wherever it isdesirable to determine the presence of minute quantities of materialswhich will absorb radio frequency energy. For example, my invention canbe used:

1. As a continuous monitor on the purity of distilled water as deliveredfrom the still.

2. For monitoring industrial processes involving the presence ofmaterials capable of absorbing radio frequency energy.

3. For monitoring industrial process where the control of pH isimportant.

4. For determining the amount of decomposition undergone by chemicalcomponents which break down upon radiation with light, heat,ultra-violet X-ray, gamma ray or other radiation into materials whichhave a capacity for absorbing radio frequency energy dilferent from thatof the parent material.

5. For determining the end point in chemical titration work.

I claim:

1. In a circuit for measuring the amount of R. F. energy absorbed by asample disposed in inductive relationship with a resonant circuit, thecombination comprising an R. F. generator of variable power output, aresonant circuit connected to the output of the R. F. generator, meansfor tuning the resonant circuit, a vacuum tube voltmeter connectedacross the resonant circuit and having a control grid and a platecircuit, means for biasing the control grid of the vacuum tube to anegative value far beyond its normal cut-oft bias, a current meterconnected in the plate circuit of the tube, means for varying the poweroutput of the R. F. generator and supplying the power to the resonantcircuit to overcome the negative bias on the vacuum tube and cause platecurrent to pass through the meter to give a maximum reading with nosample present so that the plate current will decrease substantiallywhen a sample which absorbs R. F. energy is placed in inductiverelationship with the resonant circuit.

2. In a circuit for measuring the amount of R. F. energy absorbed by asample from a capacitor, the combination comprising an R. F. generatorof variable power output, a resonant circuit containing the capacitorconnected to the output of the R. F. generator, means for tuning theresonant circuit, a vacuum tube voltmeter connected across the capacitorand having a control grid and a plate circuit, means for biasing thecontrol grid of the vacuum tube to a negative value far beyond itsnormal cut-off bias, a current meter connected in the plate circuit ofthe tube, means for varying the power output or" the R. F. generator andsupplying the power to the resonant circuit to overcome the negativebias on the vacuum tube and cause plate current to pass through themeter to give a relatively large reading with no sample present in thecapacitor so that the plate current will decrease substantially when asample which absorbs R. F. energy is placed in inductive relationshipwith the capacitor.

3. In a circuit for measuring the amount of R. F. energy absorbed by asample from an inductance, the combination comprising an R. F. generatorof variable power output, a resonant circuit containing the inductanceconnected to the output of the R. F. generator, means for tuning theresonant circuit, a vacuum tube voltmeter connected across the inductorand having a control grid and a plate circuit, means for biasing thecontrol grid of the vacuum tube to a negative value far beyond itsnormal cut-off bias, a current meter connected in the plate circuit ofthe tube, means for varying the power output of the R. F. generator andsupplying the power to the resonant circuit to overcome the negativebias on the vacuum tube and cause plate current to pass through themeter to give a large reading with no sample present in the inductanceso that the plate current will decrease substantially when; av samplewhichwabsorbs' R'. ;F. energy is.=placed .in inductive: relationshipwithi the inductance.

4. In a circuit for measuring the amount of R. F. energy absorbed by asample disposed in inductive relationship with a resonant" circuit,thecombination comprising an R. F. generator ofvariable power output, aresonant circuit-connected-to. the output of the R. F. generator, sampleholding means "disposed in the resonant circuit, means for tuning theresonant circuitya vacuum tube voltmeter connected-across the-resonantcircuit and having a control grid and a plate circuit, means forbiasingthe control grid of the vacuum tube to a negative value farbeyond its'normal cut-oif bias, a current meter connected in the platecircuit of the tube, means for varying the power output of the R. F.generator and supplying the-power to the resonant circuit to overcomethe negative bias on the vacuum tube and'cause plate current to passthrough the meterto give a maximum reading with no sample present sothat the plate current will decrease substantially when a sample whichabsorbs R. F. energy is placed in inductive relationship with theresonant circuit.

5. in a circuit for measuring the amount of R. F. energy absorbedby asample from a capacitor, the combination comprising an -R. F. generatorof variable power output, a resonant circuit containing the capacitorconnected to the output of the R. F. generator, sample holding meansdisposed in the-capacitor, means for tuning the-resonant circuit, avacuum-tube voltmeter connected across the capacitor andhaving acontrolgrid and a plate circuit, means for biasing the controlgrid of thevacuum tube to a negative value far beyond its normal cut-oft bias, acurrent meter connected in the. plate circuit of the tube, means forvarying the power output of the R. F. generator and supplyingthe powerto the resonant circuit to overcomethenegative bias-onthevacuumtube andcause plate current to pass through. the meter to give a relativelylarge reading with no sample present in the capacitor so that the platecurrent will decrease substantially when a samplewhich absorbs R. F.energy is placed in inductive relationship with the capacitor.

6. In a circuit for measuring the amount of R. F. energy aborbed by asample from a capacitor, the combination comprising an R. F. generatorcapable of variable power output, a resonant circuit containing thecapacitor connected to the output of the R. F. generator, sample holdingmeans disposed in the capacitor, means for tuning the resonant circuit,a vacuum tube voltmeter. connected across the capacitor and having acontrolv grid and plate circuit, means for biasing the control grid ofthe vacuum tube to a negative value of the order of ten times its normalcut-ofi bias, a. current meter connected'in the pla'te'circuit of thetube, means for varying the power output of the R. F. generator andsupplying the power to the resonant circuitsufficient to overcome thenegative bias on the vacuum tube and cause plate current to-pass through:the meter to give a relatively large reading with no sample present inthecapacitor-so that the plate current will decrease substantially whenthe sample which absorbs R. F energy is placed in the sample holder.

7. In a circuit for measuring the amount of R. F. energy absorbed by asample disposed in inductive relationship with a resonant circuit, thecombination comprising an R. F. generator of variable power output, aresonant circuit connected to the output of the R. F. generator, meansfor tuning the resonant circuit, means for continuously flowing thesample through an inductive relationship with the resonant circuit, avacuum tube voltmeter connected across the resonant circuit and having acontrol grid and a plate circuit, means for biasing the control grid ofthe vacuum-tube to a negative value farbeyond its normal cut-off bias, acurrent meter connectedin the plate circuit of the tube, means forvarying the power output of the R. F. generator and supplying the powerto the resonant circuit to overcome the negative bias on the vacuum tubeand'cause plate current to pass through the meter to give a maximumreading with no sample present so that the plate current will decreasesubstantially when a sample which absorbs R. F. energy is placed-ininductive relationship with the resonant circuit.

8. In a circuit for measuring the amount of R. F. energy absorbed by asample disposed in inductive relationship with a resonant circuit, thecombination comprising an R. F. generator of variable power output, avariable inductor and a capacitor forming the resonant circuit connectedto the R. F. generator output, a vacuum tube voltmeter connected acrossthe capacitor and having a. control grid and a platecircuit, means forbiasing the control grid to a negative value far beyond its normalcut-off bias, a current meter connected in the plate circuit, means forvarying the power output of the R. F. generator suflicient to overcomethe negative bias on the vacuum tube and cause plate current to passthrough the current meterto give a large reading with no sample presentso that the plate current will decrease substantially'when a samplewhich absorbs R. F. energy is placed in inductive relationship with thecapacitor.

References Cited in the file of this patent UNITED STATES PATENTS

